I am trying to set up my Jebao DP-4. I have calibrated and ran the doser to get all the air out of the line. However, after couple hours I checked the tubing and it has air coming from the input and output of the head. I ran it again to get rid of the air bubbles and it occurred once more. I looked over the tubing and it's on the fittings pretty tight on. Are the doser heads broken?